Accumulating evidence indicates that the maintenance of telomeres regulates both chromosomal stability and cell replicative lifespan. In addition, telomere biology plays critical roles in the pathogenesis of human cancer and may contribute to aging. Studies in both human and animal models indicate that telomeres and telomerase, the reverse transcriptase that sustains telomere structure, serve dual roles in oncogenesis, serving both to suppress and facilitate neoplastic transformation. In humans, constitutive telomerase expression facilitates cell immortalization, is required for the long-term growth of human tumors, and cooperates with oncogene expression to transform primary human cells to tumorigenicity. These observations support the hypotheses that telomerase overexpression is critical to cancer development and that repression of telomerase expression serves as a mechanism to suppress tumor formation. Despite this conceptual framework, we lack fundamental insights into the molecular mechanisms that regulate telomerase expression and function. In normal human cells, telomeres shorten with successive rounds of cell division, and immortalization correlates with stabilization of telomere length. These observations suggest that human cancer cells achieve immortalization, in large part, through the illegitimate activation of telomerase. However, we have recently found that the rate-limiting telomerase catalytic subunit, hTERT, is expressed in cycling primary human fibroblasts, previously believed to be devoid of telomerase activity, and that this low-level expression of telomerase controls cell proliferation. Since such cells exhibit telomere shortening with passage in culture, this unexpected observation suggests that telomerase plays critical roles in regulating cell lifespan beyond simply maintaining telomere lengths. In this application, we propose to investigate the regulation of telomerase stability and to use molecular biological, genetic, and biochemical approaches to understand the functional roles of telomerase in both normal and malignant cells. Investigating these new functions of telomerase in human cells will not only enhance our understanding of telomere biology but will also alter our perceptions of the roles of telomerase in cancer and aging. Since diagnostic and therapeutic strategies that target telomerase are under development, determining the role of telomerase in the physiology of normal human cells promises to provide critical insights into the potential specificity and effectiveness of these approaches. [unreadable] [unreadable] [unreadable]